Control of the gear shift points for changing from one gear to another in heavy duty automatic transmissions of the type used in trucks, buses and off road equipment historically have been controlled by mechanical inputs such as centrifugal governors monitoring engine speed and driveshaft speed along with throttle position and, in some instances, output shaft torque.
The shift point from transmission to transmission for any given set of conditions varies significantly due to the inherent manufacturing tolerances and the calibration of the various mechanical linkage components. The mechanical inputs activate pilot spool valves which, in turn, control main spool valves to energize and de-energize the various clutches as the transmission shifts from one gear to the next. Since the spool valves are activated mainly in an ON/OFF mode with little or no pressure regulation between the extremes, the shifts tend to be quite abrupt causing torque spikes in the transmission output and high pressure clutch slippage resulting in clutch face wear.
By electronically monitoring engine speed, output speed and torque, and throttle position, it is possible with an inexpensive microcomputer to calculate the exact shift point required for any set of conditions and to control an electrohydraulic device to energize and/or de-energize the appropriate clutches when shifting from one gear to another. The clutch for the desired gear is engaged as the clutch for the undesirable gear is being de-energized. In energizing a clutch of this type, it is necessary to first fill the clutch hydraulic cavity with hydraulic fluid at a low pressure, such as 10-20 PSI, which compresses the clutch return springs and brings the clutch faces into low pressure sliding engagement. The hydraulic volume necessary to fill the clutch can be up to 10 cubic inches and is variable within any given clutch depending on the amount of residual oil in the clutch at the beginning of fill and the amount of wear on the clutch faces and components. It is desirable to fill the clutch volume rapidly, for example, in 0.5 seconds or less, therefore, an initial high flow rate is required, for example, 10-20 gallons per minute.
Once the clutch is full, the pressure in the clutch cavity must be linearly increased from the low 10-20 PSI fill pressure to line pressure to bring the clutch faces into non-slipping high pressure engagement. Line pressure is normally high, on order of 150 PSI, but may vary from 60 PSI to 350 PSI.
After the clutch is fully applied, it is necessary to "latch" or hold the clutch in this position without an electrical signal to the electrohydraulic valve. This "latching" of the clutch is a safety measure to insure that the transmission will remain in gear in the event of electrical power loss.
To de-clutch, it is necessary to linearly reduce the pressure within the clutch from line pressure to zero.
The present invention is directed to an electrohydraulic control device to energize and de-energize clutches through electrical commands from a microcomputer.
Among the objectives of the invention are to provide an electrohydraulic control system which comprises a spool valve including a body having a bore. A spool is positioned in the bore for reciprocating movement within the bore. The valve body has an inlet, an outlet and an exhaust port. An electrohydraulic three-way normally closed pulse width modulated valve has an inlet communicating with the inlet to the valve body and an outlet communicating with one end of the spool. An electrohydraulic three-way normally closed ON/OFF valve has an inlet communicating with the outlet of the body and an outlet communicating with the other end of the spool. A spring yieldingly urges the spool toward the pulse width modulated valve. The spool is positioned so that it normally obstructs a path from the inlet to the outlet.